Aquarium Tank Tops

Aquarium Tank Automatic Top Off Tunze Osmolator Universal 3155 Auto Top Off ATO

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Aquarium Tank Automatic Top Off Tunze Osmolator Universal 3155 Auto Top Off ATO

Tunze Osmolator Universal 3155 Auto Top Off ATO

The TUNZE Osmolator 3155 is an automatic top off (ATO) system for marine and reef aquariums. The Osmolator – short for Osmotic Regulator – automatically adds freshwater to compensate for evaporation in your saltwater aquarium.

The high-tech German designed Osmolator uses a precision optical sensor to measure the water level. Water continuously evaporates from the aquarium, leaving the salts behind. This causes the salinity to increase. When you manually pour in water the salinity is suddenly decreased. This “salinity rollercoaster” stresses the fish and invertebrates by causing extreme fluctuations in osmotic pressure.

The Osmolator puts an end to this by continuously sensing the rate of evaporation (water level) and adding back the right amount of freshwater. This brings osmotic stability to the aquarium and keeps your sump and aquarium water at the right level.

Here’s what we really like about the Osmolator:

  • TUNZE invented the ATO and has been perfecting it for 30 years
  • Precision optical water sensor detects the slightest change in water level
  • Easy to install and adjust using the Magnetic Sensor Mount
  • Status lights display functions and sounds a warning in case of trouble
  • Built-in safety features prevent overfilling of the aquarium and shuts the system down
  • Comes ready to install – all you need is a reservoir or bucket

The Osmolator can be used on any aquarium, even freshwater tanks. It works when installed inside the aquarium on tanks without a sump. If you have a sump the sensor must be installed there since that is where the water level changes as it evaporates.


  • Complete kit with power supply, metering pump, hose and sensor mounting system.
  • The water level sensor has no moving parts to wear out and is not affected by suspended dirt, light or magnetic fields.
  • Optical sensor ignores gentle water movements caused by wave makers and circulation pumps.
  • An independent safety sensor with a separate power supply circuit and relay ensures reliable overflow protection.
  • A microprocessor monitors the water level in the reservoir and prevents the pump from running dry.
  • All functions are clearly displayed by colored LEDs and warning signals.
  • The Osmolator uses ultra-low voltage for complete safety.


  • Metering pump flow rate based on pumping height: 3 feet:13.7gph, 5 feet:12.4gph, 6.5 feet: 7gph, Maximum pumping height: 7.22 feet
  • Magnetic Mount System: Maximum glass thickness ½ inch
  • For indoor use only
  • 2-year warranty

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